(1) Field of the Invention
The present invention relates to a liquid crystal display device, and in particular, to a liquid crystal display device having a backlight where light emitting diodes are provided on a flexible printed circuit.
(2) Description of the Related Art
Liquid crystal display devices are used as display devices for various electronics, such as cellular phones, digital still cameras and portable information terminals. They have a backlight for radiating light in the liquid crystal display panel in compact and thin display devices, as in JP2008-170739A, light-emitting diodes LED are used as the light source for the backlight, and as shown in FIG. 1, the light emitting diodes LED are fixed on a flexible printed circuit. The flexible printed circuit has wires, not shown, so that power can be supplied to the individual light emitting diodes LED in the structure.
FIG. 2 is a plan diagram showing the relative position of a flexible printed circuit FPC having light emitting diodes LED and a light guide plate LG for guiding light emitted from the light emitting areas EA of the light emitting diodes LED. FIG. 3 is a cross sectional diagram along A-A′ in FIG. 2. A liquid crystal display panel, not shown, is provided at the bottom in FIG. 3. Thus, a reflective sheet RS is provided so as to cover the light emitting diodes LED and the light guide plate LG and reflect light coming from the bottom.
As shown in FIGS. 2 and 3 there is a space G between the light emitting diodes LED (light emitting areas EA) and the light guide plate LG. Ideally there would be no space G, but in reality there ends up being a space of 50 μm to 100 μm or more due to error in the manufacture of the parts and the assembly.
In addition, as shown in FIG. 3, the flexible printed circuit FPC, which is translucent orange, is exposed in the space G, and thus part of the light emitted from the light emitting areas EA of the light emitting diodes LED is absorbed or scattered by the flexible printed circuit FPC, as shown by the arrows, and fails to be guided into the light guide plate LG, and thus the brightness of the backlight lowers.
Furthermore, as liquid crystal display devices and their frames become thinner, light emitting diodes LED used for the backlight are becoming smaller, with a thickness t now of 0.6 mm or 0.4 mm, which is thinner than the light guide plate. Therefore, the reduction in brightness in the space G becomes more significant. The graph in FIG. 4 shows the change in the average brightness of the backlight as measured when there is a space G between the light emitting diodes LED and the light guide plate LG. The average brightness lowers by slightly less than 20% when the space G is 0.2 mm, even with light emitting diodes having a thickness t of 0.6 mm. The brightness is reduced by more than 30% for the same space G (0.2 mm) with light emitting diodes having a thickness t of 0.4 mm. Thus, it is clear that the problem with the brightness lowering due to the space G is very serious when liquid crystal display devices and their frames are thin.
In order to prevent the brightness of the display from lowering, reflective tape 10 can be pasted on the flexible printed circuit in front of the light emitting diodes LED, as shown in FIG. 5. However, there still remains a space between the light emitting diodes and the reflective tape 10, where light is absorbed by the flexible printed circuit. Furthermore, light is absorbed or scattered at the edges of the reflective tape, and thus less light enters into the light guide plate.
As shown in FIG. 6, it is also possible to prevent the flexible printed circuit FPC from absorbing light by making it white. In this case, however, the cost of the flexible printed circuit increases, and the effects of preventing the brightness from lowering are not significant, due to the low reflectance.